Pool and spa water must be treated periodically (e.g., at two-week intervals or whenever cloudiness is present) to increase the clarity of the water and reduce eye burn and chlorine odor by destroying monochloramine which may be present in the water. A commonly-used agent for this purpose is Oxone.RTM. (i.e., potassium monopersulfate) which is a strong oxidizing agent. It is commonly used as an auxiliary oxidant (i.e. shocking agent) in swimming pools and spas to reduce the organic content of the water.
Unlike chlorine-based shocking agents that super-chlorinate the water, the use of potassium monopersulfate does not increase the chlorine level in the water. Consequently, a pool does not have to be closed except for a short period of time to fully circulate the agent.
Because the potassium monopersulfate is not a disinfectant, it must be used in conjunction with a chlorine or bromine-based disinfectant. This use of chlorine in addition to the potassium monopersulfate, however, causes a problem. Because a pool or spa is usually opened shortly after the addition of the agent, it is necessary to test the levels of chlorine or bromine that are present. Unfortunately, the potassium monopersulfate in the water interferes with DPD, the most common test for these disinfectants.
The DPD (N,N-diethyl-p-phenylenediamine) method for determining free and total chlorine in water and wastewater has been widely used for many years. The DPD amine is oxidized by chlorine to two oxidation products. At a near neutral pH, the primary oxidation product is a semi-quinoid cationic compound known as a Wurster dye. This relatively stable free radical species accounts for the magenta color in the DPD colorimetric test. DPD can be further oxidized to a relatively unstable, colorless imine compound. When DPD reacts with small amounts of chlorine at a near neutral pH, the Wurster dye is the principal oxidation product. At higher oxidant levels, the formation of the unstable colorless imine is favored-resulting in apparent "fading" of the colored solution. The DPD Wurster dye color has been measured photo-metrically at wavelengths ranging from 490 to 555 nanometers (nm).
Monochloramine and dichloramine are slow to react directly with DPD at a near neutral pH. To quantify these species, the test is performed under slightly acidic conditions in the presence of iodide ion. The iodide reacts with the chloramines to form iodine as the triiodide ion (I.sub.3.sup.-). The triiodide, in turn, reacts with DPD, forming the Wurster oxidation product.
Although potassium monopersulfate interference with a free chlorine test is only slight, it produces significant interference with a total chlorine test. The total chlorine test relies on the ability of combined chlorine to convert iodide to iodine which then reacts with DPD to form a colored product. Potassium monopersulfate is also capable of making this conversion and, at a use rate of approximately 30 ppm, the interference is severe. A water sample containing no chlorine and 30 ppm potassium monopersulfate will typically produce a false reading of 5 ppm chlorine.
Previous attempts to eliminate potassium monopersulfate have involved the addition of EDTA (ethylenediaminetetraacetic acid). However, when EDTA is combined with the DPD, its ability to mask the monopersulfate is diminished. If additional EDTA is added, it is not soluble in a DPD solution (which must be stored at a very low pH to maintain stability). Only a limited amount, 0.5% by weight, of EDTA is soluble in a low pH DPD solution. This small amount of EDTA is not sufficient to eliminate the monopersulfate before it can react with the DPD.
There has not heretofore been provided a reliable test for chlorine in water (such as pool and spa water) when using potassium monopersulfate as an oxidizing agent.